There has long been a requirement to detect small cracks and defects present on the faying surfaces of aircraft wing skin material present around fastener holes formed therein. This procedure is carried out with the fastener installed in the fastener opening. Aircraft fastener holes are prone to developing cracks that can propagate and result in serious structural failure.
The current system has been in service for many years, but is now obsolescent and no longer maintainable. Its drawbacks include a mechanically-complex scanner that requires the physical rotation of two conventional single-element ultrasonic probes (sequentially—one in each direction around the entire circumference of the hole) that requires a long inspection time of up to several minutes per fastener hole. Further, it is necessary to center the scanner very precisely over the hole, which contributes to the length of the inspection, particularly because the scanner is of such weight that it must be supported by a floor stand when conducting under-wing inspections.
One known way to improve upon this system is to utilize a matrix of ultrasonic elements arrange in a conical configuration encircling the fastener head. This arrangement permits deflection of the ultrasonic beams it forms in three dimensions, and adapts to different hole diameters and skin thicknesses. Initial approaches to this solution were described in the following three papers: (1) Inspection of Fastener Holes Using Ultrasonic Phased Arrays by Moles, Lamarre, Selman, Miller and Herzog presented at the 2000 USAF Aircraft Structural Integrity Program Conference, December 05-07 in San Antonio, Tex.; (2) Three-Dimensional Imaging of Fastener Holes Using Ultrasonic Phased Arrays by Lupien, Moles, Selman, Miller and Herzog at the poster session for Review of Progress in Quantitative NDE, Jul. 17-21, 2000 in Ames, Iowa; and (3) Three-Dimensional Ultrasonic Phased Array Imaging for Fastener Inspections by Lupien, Moles, Selman, Miller and DoD/FAA/NASA Conference on Aging Aircraft Conference, May 15-18, 2000 in St. Louis, Mo.
Two later and more detailed descriptions of the technology are contained in (1) Inspection of Aircraft Fastener Holes Using a Conically Shaped Multi-Element Phased Array Probe by Selman, Miller, Moles, Dupuis and Herzog presented at the 28th Annual Review of Progress in Quantitative Nondestructive Evaluation Conference held in Brunswick, Me. Jul. 29-Aug. 3, 2001 and subsequently published in Volume 21A of the proceedings of the conference; and (2) A Novel Fastener Inspection Method Using an Ultrasonic Phase Array Probe by Selman, Miller, Moles, Dupuis and Herzog at the Aging Aircraft 2001 Conference, Sep. 10-14, 2001.
Essentially, phased arrays ultrasonic systems generate focused beams by controlling the timing of the emission of sound waves generated from a plurality of separately spaced piezoelectric elements. Not only can focused beams of ultrasonic waves be formed, but such beams may be directed within a volumetric working space to probe for discontinuities in the media transmitting the sound waves. Defects beneath the surface of an aircraft wing surrounding a fastener are detectable on the basis of sonic echoes that are returned or deflected from such discontinuities. As phased array beams are generated electronically, electronic scanning permits very rapid inspections of structural components that have uniform geometries.
A need exists for a handheld, lightweight, portable crack detection system that can be rapidly positioned over fasteners, and then can rapidly detect faying surface cracks in the first layer around the base of a fastener hole with the fastener installed.
Objects of the invention are therefore to provide a means for positioning a phased array ultrasonic probe centrally over a cylindrical hole in a surface to be tested; and means to detect defects in the materials surrounding the hole efficiently and reliably using phased array ultrasonic technology.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims that conclude this Specification.